A voltage nonlinear resistance element formed by adding a rare earth element, Co and B to a main component of ZnO is extensively employed for protecting electronic equipment from abnormal or surge voltages. The ZnO voltage nonlinear resistance element is manufactured through the steps of mixing, granulating, molding, and baking of the raw material as for conventional electronic ceramic devices. The conventional voltage nonlinear resistance element manufacturing method, however, suffers from several disadvantages. In mass-producing the voltage nonlinear resistance element, the characteristics of the elements may fluctuate within the same lot of materials. In the case of manufacturing voltage nonlinear resistance elements relatively large in outside diameter, different portions of each element often show different characteristics. These are serious problems to be solved in the manufacturing of voltage nonlinear resistance elements.
The present inventors have conducted intensive research on the above-described manufacturing steps in order to solve the related problems, and found that the baking step most seriously affects the characteristics of the element. As was described above, in manufacturing the ZnO voltage nonlinear resistance element, various elements are added, as auxiliary components, to a main component. Of these additive elements, boron (B) is used in the form of B.sub.2 O.sub.3. The melting point of B.sub.2 O.sub.3 is much lower than the baking temperature, and the vapor pressure thereof is high. Therefore, it has been found through analysis of the compositions of baked voltage nonlinear resistance elements that boron evaporates readily during the baking operation, and depending on the baking conditions, the distribution of boron becomes considerably non-uniform with respect to the predetermined content of boron.
Boron (B) is used to grow the crystal of ZnO. Therefore, if B evaporates partially during baking, then in the element the grain size becomes much different from the expected size. Accordingly, since the degree of evaporation of boron among the elements may differ from one another, the values V(1 mA) of the elements may also be much different from one another. The term "V(1 mA)" is intended to mean the voltage developed across the terminals of the element when a current of 1 mA flows in the element, and it is a typical measurement of the characteristic values of ZnO voltage nonlinear resistance elements. In the case of a voltage nonlinear resistance element relatively large in diameter, evaporation of boron forms different characteristics in one and the same element, thus adversely affecting the long wave tail surge withstanding data.
This difficulty may be overcome by a method in which in order to prevent the evaporation of boron, the mold is enclosed in a sheath box, i.e., it is baked in a closed container. However, this method is still disadvantageous in that the peripheral portion of the baked element will have decreased resistance, as a result of which current flows collectively in the peripheral portion, and the long wave tail surge withstanding data is decreased.